Specific Aim 1: Determine the heterogenic distribution of positive force feedback in the cat hindlimb. Proprioceptive feedback of mechanical information, force and length, plays an important role in maintaining balance, tuning locomotion and controlling joint stiffness. Force feedback is broadly distributed across muscles and may play an important role in controlling whole limb behavior by providing compensation for nonuniform segment inertias and interaction torques. Specific Aim 2: Develop a 3-dimensional musculoskeletal model of the cat hindlimb. Few muscles in the cat hindlimb have been found to operate in a single plane. To represent accurately the interactions between muscles thus requires a 3-D model. In a feedback-intact system, muscle mechanics can be somewhat simplified, thus emphasizing the mechanical and neural interactions between muscles. Specific Aim 3: Incorporate heterogenic reflex feedback into the musculoskeletal model. Understanding the mechanisms by which length and force feedback simplify muscle control may, in turn, simplify the design of systems to bypass traumatically or pathologically damaged segments of the nervous system. Such a system may provide a framework for investigating the rich literature surrounding reflex mechanisms.